This disclosure relates generally to the field of optics and, more specifically, to systems and methods for an optical coupling between a circular-cross section beam and a high aspect ratio cross section beam.
Gain media based on optical fibers (fiber lasers and amplifiers) are of intense interest due to a broad range of performance features, including high efficiency, robust single-mode output, high reliability, compact coiled packaging, large surface-area-to-volume ratio for favorable thermal performance, and an all-fiber architecture without any free-space optics and hence no requirement for a rigid optical bench. Over the past decade, output of fiber lasers have been increased several orders of magnitude, from the watt-level in the mid-1990's, to multi-kW powers over the past few years.
Several free-space optics methods are known to be effective for changing a beam format from a collimated round shape to a collimated elliptical shape. A single cylindrical lens is often used for free-space coupling of a circular beam-into a planar waveguide, which is located at a focus or image plane of the lens. Various alternatives of this basic approach have been employed, some involving three cylindrical lenses rather than two spherical lenses and one cylindrical lens. But the basic functioning of the lenses remains unchanged. The disadvantage of this free-space approach is that it is bulky, requiring at least three lenses set in a row. Moreover, an integrated version of this technical approach is not possible, thus resulting in increased manufacturing costs.
The most common method of beam re-formatting involves a telescope that usually consists of two lenses, which are separated by a distance equal to a sum of their focal lengths, and with the lenses sharing a common focal plane. The cylindrical telescope, which incorporates two cylindrical lenses with parallel orientation of the axes, is often used to reformat a beam with respect to one axis only. In principle, such a telescope does not affect the beam size and collimation of the other axis. Moreover, such a free-space telescope is not an integrated, all-glass design. Nevertheless, one should note that fiber-based telecom components, such as isolators, actually incorporate very small free-space components such as gradient index (GRIN) lenses, polarizers, optical filters, etc., in a compact robust package that can pass the rigorous Telcordia acceptance test standards. Hence, in the context of the present disclosure it is useful to consider what a fiber-based free-space cylindrical signal coupler would look like.
Assume that cylindrical imaging is used in the narrow fast-axis plane and simple collimation is used in the wide slow-axis plane, and assume further that the circular input fiber has a core diameter of d=20 μm and the output fiber has a rectangular core with dimensions of 20 μm×2.5 mm. The beam divergence angle is ˜λ/d; for a wavelength of 1 μm, this yields an angle of 50 mrad for the circular input fiber. Consequently, a propagation distance of >5 cm is required for this beam to expand to the point that it matches the 2.5-mm wide dimension of the output fiber. Clearly this length is unacceptable. One could certainly replace the single slow-axis lens with a lens pair that would require much less propagation distance, but as the distance is reduced the lens focal lengths must decrease correspondingly. For example, suppose we require the total length of the coupler to be a maximum of 100 mm, and assume a magnification of 125 (which converts 20 μm into 2.5 mm). If we set the longer focal length to occupy essentially the full 100 mm, the shorter focal length must be 0.8 mm to yield the desired magnification. But this type of microlens is difficult to make with high quality, particularly as a cylindrical lens, and it would be very sensitive to alignment. The main point is that free-space optics not only do not meet the requirements for being all-glass, but such an approach would also be very risky even to make a free-space coupler at an acceptable length of ˜100 mm. It would be preferable to use shorter couplers, perhaps 10 mm long, which would not be available using this lens arrangement.
A common method of matching a circular-fiber mode to a planar waveguide mode is to use “butt coupling.” A polished (or well-cleaved) fiber tip is aligned to point along the waveguide axis, and is set as close as possible to a polished end face of the waveguide. An integrated version of butt coupling exists, too, where the fiber is fused to the planar waveguide end face. However, butt coupling does not offer any way to vary the sizes or divergence angles in the two transverse dimensions. Consequently, if a butt-coupler were to be designed to match the fast-axis dimension and divergence to that of a semi-guiding high-aspect ratio core (SHARC) gain fiber, the signal would grossly underfill, and be highly divergent, in the slow-axis dimension, resulting in high order multimode excitation of the SHARC fiber. Conversely, if the butt coupling were designed to match the parameters of the slow-axis dimension, the signal would grossly overfill the fast-axis direction, and this would significantly lower the system efficiency.
Two all-glass integrated approaches are known for re-formatting optics. The first one refers to tapering the diameter and/or re-shaping the core of a signal fiber. Fiber tapering with proportional scaling of both transverse dimensions of the core and cladding, while maintaining a circular cross-section, is a common practice in fiber optics. Core shape transformation from a circle to a rectangle has been demonstrated as well, in photonic-crystal fibers, for example. This particular method helps to transform a round core to a moderate aspect ratio rectangle, about 1:5. However, the feasibility of employing the same approach to transform a round core to a very high aspect ratio core of interest for SHARC is highly questionable. A drawback of adiabatic re-shaping is that the rate at which the transverse dimensions change with length along the fiber must be very slow to minimize radiative loss into the cladding. This requirement translates into very long lengths for shape-transforming tapers with tight manufacturing tolerances if the lowest-order mode structure and polarization are to be conserved. Tapered planar channels of variable width are also known. But they are used to interconnect two planar channels having different widths, which are located at a common solid substrate, not a fiber. Moreover, a ribbon-shaped flexible planar waveguide tapered along the slow-axis direction has also been proposed. However, this approach serves for reformatting incoherent light, and is not compatible with the requirement to maintain single-mode, collimated operation along the slow-axis direction.
The second integrated approach for re-formatting a guided beam, the use of a GRIN lens, is widely employed within the commercial market for fiber optic components. A GRIN lens is designed as a thin glass rod with mm-scale transverse dimensions. An optical fiber can be fused to one or both flat GRIN-lens end faces. The focusing effect in common GRIN lenses occurs because of a transverse variation of the glass refractive index from the axis to the periphery; this variation is typically achieved by diffusing dopants into a cylindrically shaped glass rod, with a resulting radial gradient in the dopant concentration and, hence, the refractive index. The transverse index gradients results in transverse variations of the optical path for light rays, which is needed for focusing light. GRIN lenses can serve various optical functions as a single compact component. A GRIN lens rod having a “quarter-pitch” length allows a diverging fiber-mode beam to expand into a circular beam and provides its collimation. Doubling the length to a “half pitch” provides imaging of the input end face of the GRIN rod to its output end face.
Commercial GRIN lenses are made with a circular cross section. They collimate and expand a fiber mode in both transverse dimensions simultaneously, and, hence, they cannot be used for changing the beam aspect ratio, as is required for coupling to a SHARC fiber. Meanwhile, a one-dimensional version of a custom GRIN lens integrated into a circular fiber has been proposed. The corresponding planar GRIN lens differs from a conventional cylindrical lens in the following way: the refractive index changes gradually, in a parabolic manner, with respect to only one transverse coordinate.
What is needed is a monolithic high-power signal coupler to transform a circular beam to an elliptical beam between a circular and SHARC fiber, that provides a robust all-fiber amplifier architecture with a long operating lifetime, without extra optical surfaces to be kept clean, and without expensive “fast” optics or precise alignments or the need to maintain a high degree of alignment precision over a wide range of operational thermal and vibrational environments.